JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B02204, doi:10.1029/2003JB002845, 2004
Late Cretaceous tectonic history of the Sierra-Salinia-Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California Ronald C. Schott1 and Clark M. Johnson Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin, USA
James R. O’Neil Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA Received 14 October 2003; revised 5 November 2003; accepted 17 November 2003; published 11 February 2004.
[1] Upper Cretaceous and Paleogene conglomerates in the Gualala basin of northern California likely preserve the westernmost record of sedimentation in California during the interval between cessation of Sierran batholith magmatism and inception of Neogene transform activity on the San Andreas fault system. Detailed chemical and H, O, Sr, Nd, and Pb isotope analyses of conglomerate clasts, as well as U/Pb zircon ages, identify two source types: (1) evolved granite, like those found in the Salinian block or eastern Sierra Nevada batholith, and (2) primitive oceanic arcs that were restricted to the western portions of the Mesozoic arc system. The provenance of the Gualala clasts can be entirely explained through derivation from the Sierra-Salinian-Mojave portion of the Mesozoic Cordilleran arc, and do not support models for extensive large-scale translation of the basin from low latitudes along a pre-Neogene transform fault system. Contemporaneous deposition of the contrasting lithologies in the Gualala basin is interpreted to reflect tectonic juxtaposition of the source terranes approximately 80 m.y. ago. Tectonic processes responsible for juxtaposition of the two source regions in the Late Cretaceous most probably involved zones of lithospheric weakness that later localized the San Andreas fault system in the Neogene. Chemical, isotopic, and age data tie the Gualala basin to North American continental basement in the Mojave-Salinian segment of the Mesozoic Cordilleran arc and imply minimal pre-Neogene translation of the basin contrary to the low latitude origin for the basin inferred from previous paleomagnetic and paleontologic studies. INDEX TERMS: 1035 Geochemistry: Geochronology; 1040 Geochemistry: Isotopic composition/chemistry; 1065 Geochemistry: Trace elements (3670); 8105 Tectonophysics: Continental margins and sedimentary basins (1212); KEYWORDS: isotope, provenance, cordillera Citation: Schott, R. C., C. M. Johnson, and J. R. O’Neil (2004), Late Cretaceous tectonic history of the Sierra-Salinia-Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California, J. Geophys. Res., 109, B02204, doi:10.1029/2003JB002845.
1. Introduction North America. More controversial are interpretations of thousands of kilometers of displacement suggested by some [2] Sedimentary basins in orogenic belts record tectonic paleomagnetic and paleontologic studies [e.g., Kanter and events that may manifest themselves on the surface, which Debiche, 1985; Elder et al., 1998]. A significant unresolved may be distinct from those that are preserved in deep crustal aspect of the latter hypotheses is the documentation of environments. In some cases, sedimentary basins may faults along which such large displacements could be record the effects of large-scale translation of exotic terranes accommodated. [e.g., Cowan, 1994; Dickinson and Butler, 1998]. Many [3] The Gualala basin offers a unique opportunity to exotic terranes of western North America have demonstra- address the tectonic evolution of the Cordillera of California ble and well-accepted post-Cretaceous displacements of and of the role of the faults that offset it. Currently situated tens to hundreds of kilometers with respect to cratonic adjacent to the San Andreas fault in northern California (Figure 1), the basin is clearly offset from its location of origin by motion along the San Andreas fault system in the 1Now at Department of Geology and Physics, Lake Superior State University, Sault Sainte Marie, Michigan, USA. Neogene [e.g., Ross, 1984; Graham et al., 1989; Powell, 1993]. Pre-Neogene translation may have occurred as well. Copyright 2004 by the American Geophysical Union. Some workers have suggested that the basin was originally 0148-0227/04/2003JB002845$09.00 located hundreds of kilometers south of its current position
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Figure 1. Current location in California of the displaced Salinian block (stippled pattern) and Gualala basin (GB), as well as the Eagle Rest Peak (ERP) mafic basement terrane and its offset counterparts at Gold Hill (GH) and Logan Quarry (LQ). Faults: SAF, San Andreas fault; GF, Garlock Fault; SGHF, San Gregorio-Hosgri fault; RRF, Reliz-Rinconada fault; SNF, Sur-Nacimiento fault. Cities: SF, San Francisco; LA, Los Angeles; FR, Fresno; BK, Bakersfield. and adjacent to the northern Salinian block and southern San Peak (ERP) and its outliers at Gold Hill (GH) and Logan Joaquin Valley [e.g., Wentworth, 1966; Ross et al., 1973; Quarry (LQ), rocks that have been debated as possible Kistler et al., 1973; James et al., 1993; Schott and Johnson, source materials for mafic conglomerate clasts at Gualala. 1998a, 1998b, 2001], while others place it thousands of Identifying these sources would independently constrain kilometers south of its current position [e.g., Kanter and offset on the San Andreas fault in central California [Ross, Debiche, 1985; Elder et al., 1998]. Detailed discussions of 1970; Ross et al., 1973; Kistler et al., 1973; James et al., the contrasting hypotheses for the origin of the basin are 1993]. presented by Burnham [1998] and Schott [2000]. [4] Because the sedimentary record of the Gualala basin spans almost 30 m.y. during the Late Cretaceous and early 2. Depositional Setting and Age Paleogene, sedimentary debris within it record pre-Neo- [5] Upper Cretaceous and early Tertiary sedimentary gene tectonic events. In this paper we present new geo- rocks in the Gualala basin include conglomerates, sand- chemical and isotopic data that bear on the provenance of stones and mudstones that were deposited as a turbidite fan- conglomerate clasts from the Gualala Formation. These channel complex at bathyal depths in a forearc tectonic clasts are of two types, gabbroic and rhyolitic/granitic, in setting [Wentworth, 1966; Loomis and Ingle, 1994]. places mixed together in the same beds. The present study Wentworth [1966] assigned Upper Cretaceous and lowest encompasses both the gabbroic clasts that have been the Paleocene rocks to the Gualala Formation and subdivided it focus of study by previous workers [Ross, 1970; Ross et into two members on the basis of conglomerate clast al., 1973; Kistler et al., 1973; James et al., 1993], as well and sandstone composition (Figure 2). The Stewarts as rhyolitic and granitic clasts [Schott and Johnson, 1998a], Point Member is characterized by felsic volcanic and plu- which have received less attention. The focus here is on the tonic clasts in a matrix of K-feldspar arkose, whereas the Late Cretaceous to Paleocene section and is complemented Anchor Bay Member contains a mafic clast assemblage in a by previous work on the overlying Eocene German Rancho plagioclase arkose (<5% K-feldspar) [Wentworth, 1966]; the Formation [Schott and Johnson, 2001]. New data are also two members are interfingered and hybrid felsic-mafic presented for the mafic basement terrane at Eagle Rest conglomerate clast suites occur near their mapped contacts.
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Figure 2. Generalized geologic map and stratigraphic section of the Gualala Formation (modified from Wentworth et al. [1998]). Conglomerate sample locality key: A, Black Point (KCGR clasts only); B, Smuggler’s Cove (KCGR clasts only); C, Sea Ranch Stable/Pebble Beach (interfingered and hybrid conglomerate beds, KCGR and JOGD clasts); D, Anchor Bay (JOGD clasts only); E, Havens Neck (JOGD clasts only). Localities and lithologies for clasts are given in Table 1.
The Upper Cretaceous section sits structurally above spilitic chemical analyses (Table 3), and isotopic compositions basalt of uncertain tectonic affinity [Phillips et al., 1998], (Table 4). and Wentworth et al. [1998] conclude that the contact between these units is a low angle or ‘‘detachment’’ fault. 3.1. Upper Cretaceous Conglomerate Clasts Stratigraphically above the Gualala Formation lies the [8] Upper Cretaceous conglomerate clasts can be broadly Paleocene to lower Eocene German Rancho Formation subdivided into two suites that are judged to be continental (Figure 2), another section of turbidites that are dominated or oceanic on the basis of their age and isotopic composi- by K-feldspar arkose and contain conglomerates of marked- tions. This source distinction corresponds to the composi- ly different composition [Schott and Johnson, 2001]. tional and mapped field designation of the Stewarts Point [6] Microfossils and sparse macrofossils constrain a and Anchor Bay Members of the Gualala Formation as Campanian age for the lowest Stewarts Point strata and a originally delineated by Wentworth [1966]. Clasts of conti- Maastrichtian age for the majority of the section [Loomis nental origin (designated KCGR) are most commonly and Ingle, 1994; Elder et al., 1998; Wentworth et al., 1998]. rhyolitic or granitic (often porphyritic) and have a range Conglomerates in the uppermost Anchor Bay Member of mid-Cretaceous crystallization ages, and their isotopic contain clasts and fragments of Late Cretaceous, shallow compositions suggest an origin on the eastern (continental) water macrofossils [Elder et al., 1998] in a matrix that side of the Cretaceous magmatic arc of California. In contains Paleocene foraminifera [McDougall, 1998], sug- contrast, clasts of oceanic origin (designated JOGD) have gesting that Gualala Formation sedimentation spanned the gabbroic to quartz diorite compositions, Late Jurassic crys- Cretaceous-Tertiary boundary. tallization ages, and isotopic compositions that are charac- teristic of an oceanic source. 3.1.1. Ages 3. Results [9] U/Pb age systematics of zircon from KCGR clasts are [7] Chemical and isotopic compositions were determined complex and indicate a significant amount of inherited on whole rock samples of conglomerate clasts, and U/Pb (Precambrian?) Pb in some zircons, as well as Pb loss zircon ages were determined on zircons separated from the (Table 2). None of the zircon fractions from the KCGR clasts clasts, following methods discussed by Schott [2000]. yield concordant ages (Figure 3), where inherited Pb is both a Tables 1–4 provide sample localities, interpreted ages, and major (samples KCGR-1, KCGR-8, and KCGR-11) and a rock descriptions (Table 1), U/Pb zircon ages (Table 2), minor (samples KCGR-4, KCGR-9, and KCGR-15) compo-
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Table 1a. Upper Cretaceous Gualala Formation Conglomerate Clast Sample Localities, Interpreted Ages, and Petrography Field Latitude Longitude Interpreted U/Pb Zircon Rock Classification Sample Sample North West Locality Crystallization Age,b Ma and Texturea Jurassic Oceanic Gabbro and Diorite Clasts (JOGD) JOGD-1 CJ-826-1A 38 4803800 123 3504900 Haven’s Neck 150b qtz hbd gabbro JOGD-2 CJ-826-1C 38 4803800 123 3504900 Haven’s Neck 150b hbd cpx gabbro JOGD-3 RS92GU5 38 4800800 123 3405800 Anchor Bay 150b hbd qtz gabbro JOGD-4 RS96GU2 38 4800800 123 3405800 Anchor Bay 150b hbd qtz gabbro JOGD-5 RS92SP2 38 4303300 123 2805400 Shell Beach 145 ± 1 hbd qtz gabbro JOGD-6 RS92SP13 38 4303300 123 2805400 Shell Beach 150b mus biot granodiorite JOGD-7 RS92SP14 38 4303300 123 2805400 Shell Beach 150b cpx gabbro JOGD-8 RS92SP16 38 4303300 123 2805400 Shell Beach 145 +1/ 4 hbd qtz cpx gabbro JOGD-9 RS92SP26 38 4303300 123 2805400 Shell Beach 158.5 ± 0.5 hbd qtz cpx gabbro JOGD-10 CJ-829-2A 38 4302300 123 2803100 Knipp 150b cpx gabbro JOGD-11 CJ-829-2D 38 4302100 123 2802900 Knipp 150b hbd qtz gabbro JOGD-12 CJ-829-3A 38 4303400 123 2804500 Knipp 150b hbd qtz gabbro JOGD-13 RS92SP27 38 4204600 123 2702500 Sea Ranch Stable 150b hbd qtz gabbro JOGD-14 RS92SP30 38 4204600 123 2702500 Sea Ranch Stable 144 ± 2 hbd gabbro JOGD-15 RS92SP31 38 4204600 123 2702500 Sea Ranch Stable 150b hbd gabbro JOGD-16 RS92SP32 38 4204600 123 2702500 Sea Ranch Stable 150b hbd qtz gabbro JOGD-17 ABc-1 NR NR Anchor Bay 150b hbd qtz gabbro JOGD-18 SR-3 NR NR Sea Ranch Stable 150b hbd qtz gabbro
Cretaceous Continental Granite and Rhyolite Clasts (KCGR) KCGR-1 RS92SP15 38 4303300 123 2805400 Shell Beach 85 ± 5 hbd biot granodiorite KCGR-2 CJ-829-3A 38 4303400 123 2804500 Knipp 100b hbd biot granodiorite KCGR-3 CJ-829-4B 38 4302500 123 2803300 Knipp 100b equigranular hbd biot granite KCGR-4 RS92SP40 38 4204600 123 2702400 Sea Ranch Stable 101 ± 1 equigranular hbd biot granite KCGR-5 RS92SP42 38 4201200 123 2604900 Smuggler’s Cove 100b biot granite KCGR-6 RS92SP43 38 4201200 123 2604900 Smuggler’s Cove 100b biot granite KCGR-7 RS92SP45 38 4201300 123 2604900 Smuggler’s Cove 101 ± 2 biot granite KCGR-8 RS92SP54 38 4201200 123 2604900 Smuggler’s Cove 100b rhyolite KCGR-9 RS92SP6 38 4004400 123 2505100 Black Point 125 ± 4 biot granite KCGR-10 RS92SP7 38 4004400 123 2505100 Black Point 100b medium-grained, equigranular biot granite KCGR-11 RS92SP8 38 4004400 123 2505000 Black Point 100 ± 5 biot granite KCGR-12 RS92SP10 38 4004400 123 2505000 Black Point 100b rhyolite KCGR-13 RS92SP55 38 4005500 123 2600100 Black Point 97 ± 2 coarse-grained, equigranular biot granite KCGR-14 RS92SP56 38 4005500 123 2600100 Black Point 100 ± 2 coarse-grained, equigranular biot granite KCGR-15 RS92SP58 38 4005500 123 2600100 Black Point 106 ± 2 biot granite KCGR-16 RS92SP60 38 4005500 123 2600100 Black Point 100b biot granite KCGR-17 RS92SP62 38 4005500 123 2600100 Black Point 95 ± 2 biot granite KCGR-18 RS92SP70 38 4005500 123 2600100 Black Point 98 ± 2 coarse-grained, equigranular biot granite aTexture of JOGD clasts is medium-grained, equigranular unless otherwise noted. Texture of KCGR clasts is porphyritic with aphanitic groundmass unless otherwise noted. Varietal minerals listed in order of increasing abundance. Abbreviations are biot, biotite; cpx, clinopyroxene; hbd, hornblende; qtz, quartz. bAssumed age is based on dated clasts of similar composition and texture; there is insufficient zircon yield for U/Pb age determination. nent. In contrast, two other samples (KCGR-13 and James et al. [1993] previously dated three mafic clasts from KCGR-18) yield near-concordant fractions (Figure 3) that the Anchor Bay area that yielded slightly discordant zircon have a wide range of 238U/206Pb* ages (5–11 m.y.). The fractions that have 238U/206Pb* ages ranging from 147 to zircon fractions from these two clasts have high U contents 165 Ma; two of these clasts were nearly concordant at 162 to (1000–1400 ppm U) and have lost Pb. Crystallization ages 165 Ma. James et al. [1993] interpreted their ages to be of the provenance terrane(s) span a broad range of the mid- minimum ages, on the basis of the implicit assumption that Cretaceous (circa 125 Ma to circa 85 Ma), although a uncertainties in age estimates are more likely to reflect Pb majority of rocks appear to have crystallized around 100 ± loss rather than inheritance in the mafic samples. Although it 5 Ma. The average age of 100 Ma corresponds to the time is possible that all of the circa 145 Ma zircon fractions dated of a major magmatic pulse in the California batholiths in this study represent Pb loss from a circa 160 Ma crystal- [e.g., Stern et al., 1981; Chen and Moore, 1982; Saleeby et lization age, we feel that the clustering of ages at circa160- al., 1987; James, 1992]. 165 and circa 145 Ma reflects a true bimodal age distribution [10] Four of the JOGD clasts yielded sufficient zircon for for gabbroic rocks within the provenance terrane. This age U/Pb age determinations (Table 2), and these clasts comprise distribution is similar to that observed in the Coast Range relatively evolved quartz-bearing gabbros or diorites that Ophiolite (CRO) [Hopson et al., 1981]. The lack of signif- might also be characterized as mafic tonalites. Three of these icant inherited components for the JOGD clasts is consistent clasts (JOGD-5, JOGD-8, and JOGD-14) have multiple with their oceanic character (below). concordant or near concordant fractions that imply crystal- 3.1.2. Petrography and Geochemistry lization ages at or near the Jurassic-Cretaceous boundary [11] Major element compositions of the KCGR clasts (144 ± 2 Ma) (Figure 4a). Zircons from the fourth clast reflect the highly evolved nature of their source bodies (JOGD-9) yielded a single concordant age of 158.5 ± 1 Ma. (Table 3); many KCGR clasts have >75 wt % SiO2. Because
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Table 1b. New Samples From the Mafic Basement Terrane at Eagle Rest Peak, Gold Hill, and Logan Quarry: Sample Localities, Interpreted Ages, and Petrography Interpreted U/Pb Zircon Rock Classification Sample Field Sample Latitude North Longitude West Crystallization Age, Ma and Texturea Eagle Rest Peak (ERP) ERP-1 RS92ER1 34 5503600 119 1405700 >160b fine-grained amphibolite ERP-2 CJ-40A 34 5601900 119 1200700 160c fine-grained websterite ERP-3 CJ-41A 34 5601600 119 1203800 160c hbd gabbronorite ERP-4 CJ-41A-A 34 5601600 119 1203800 160c hbd gabbro ERP-5 CJ-42A 34 5700100 119 1302400 160c ol gabbro ERP-6 CJ-42B 34 5700100 119 1302400 160c anorth gabbro ERP-7 CJ-43A 34 5603700 119 1304900 160c hbd qtz gabbro ERP-8 CJ-43C 34 5603700 119 1304900 160c qtz hbd gabbro ERP-9 RS92ER2 34 5601500 119 1202800 160c hbd gabbro ERP-10 RS92ER9 34 5602200 119 1200500 160c foliated hbd cpx gabbro ERP-11 RS92ER15 34 5600700 119 1301200 159 ± 2 hbd qtz gabbro ERP-12 RS92ER18 34 5503400 119 1402500 160c websterite ERP-13 RS92ER28 34 5604300 119 1304700 160c hbd cpx gabbro ERP-14 RS92SC1 34 5504700 119 1600900 157 +4/ 1 hbd tonalite ERP-15 RS92SC7 34 5504100 119 1603500 160c hbd tonalite ERP-16 CJ-40B 34 5601900 119 1200700 160c qtz vein ERP-17 CJ-42BQ 34 5700100 119 1302400 160c qtz vein ERP-18 DR-1169A NR NR 160c hbd qtz gabbro ERP-19 DR-1161C NR NR 160c hbd qtz gabbro
Gold Hill (GH) GH-1 CJ-21 35 4904800 120 2100000 160c cpx hbd qtz gabbro GH-2 CJ-22 35 4904300 120 2100100 160c hbd qtz gabbro GH-3 CJ-24 35 4903800 120 2005700 160c hbd qtz gabbro GH-4 CJ-25 35 4903700 120 2005500 160c hbd qtz gabbro GH-5 CJ-27 35 4903300 120 2005400 160c hbd qtz gabbro GH-6 CJ-28 NR NR 160c cpx hbd qtz gabbro GH-7 CJ-29 NR NR 160c hbd qtz gabbro GH-8 SH-4 NR NR 160c hbd qtz gabbro GH-9 CJ-GH-1 NR NR 160c hbd qtz gabbro GH-10 RS92CV5 35 4904800 120 2005300 160c hbd qtz gabbro GH-11 RS92CV6 35 4904500 120 2005600 160 ± 1 hbd qtz gabbro GH-12 CJ-24V 35 4903800 120 2005700 160c qtz vein GH-13 CJ-25VA 35 4903700 120 2005500 160c fsp vein GH-14 CJ-25VB 35 4903700 120 2005500 160c fsp vein GH-15 DR-139 NR NR 160c anorth gabbro GH-16 DR-1062 NR NR 160c hbd qtz gabbro
Logan Quarry (LQ) LQ-1 CJ-IUF NR NR 160c hbd gabbro LQ-2 CJ-2 36 5205100 121 3501500 160c hbd qtz gabbro LQ-3 CJ-LQ-1 NR NR 160c hbd qtz gabbro LQ-4 CJ-IF NR NR 160c hbd qtz gabbro LQ-5 CJ-3 NR NR 160c hbd qtz gabbro LQ-6 SH-6 NR NR 160c hbd qtz gabbro LQ-7 SJB-1 NR NR 160c hbd qtz gabbro LQ-8 RS92CH1 36 5305700 121 3604700 158 ± 2 hbd qtz gabbro LQ-9 RS92SJ1 36 5103800 121 3302300 160c hbd gabbro LQ-10 DR-588A NR NR 160c hbd qtz gabbro aTexture is medium-grained equigranular unless otherwise noted. Varietal minerals listed in order of increasing abundance. Abbreviations are ol, olivine; cpx, clinopyroxene; hbd, hornblende; qtz, quartz; fsp, feldspar; NR, not recorded. bThis is intruded by and therefore older than ERP mafic complex; insufficient zircon yield for U/Pb age determination. A 160 Ma age of ERP-GH-LQ mafic terrane is assumed for calculating initial isotope ratios. cThis is interpreted age for ERP-GH-LQ mafic complex; there is insufficient zircon yield for U/Pb age determination. textures of many of these clasts indicate a shallow depth of [12] The JOGD clasts span a wide compositional range and crystallization, it is possible that some silica was added have geochemical characteristics that are similar to those of a either by hydrothermal alteration or during weathering and primitive oceanic arc (Table 3). The most mafic clasts have diagenesis. However, on a plot of silica versus total alkalis <50 wt % SiO2, contain no modal quartz, and commonly (Figure 5), it can be seen that total alkali contents are similar have cumulate textures. More evolved JOGD clasts have to those of other high-silica granites from the California intermediate silica contents (55–65 wt % SiO2) and sparse to batholiths, suggesting little alkali mobility. The KCGR abundant modal quartz. The oceanic nature of the JOGD clasts have relatively high K2O contents (3.0–5.5 wt %) clasts is well illustrated by FeO*/MgO-TiO2 variations that are more common in igneous rocks that were intruded (Figure 6), where lack of enrichment in TiO2 with increasing into the eastern (continental) side of the mid-Cretaceous FeO*/MgO ratio is characteristic of many volcanic arcs [e.g., batholiths of California [e.g., Dodge et al., 1970]. Miyashiro, 1973]. Only one JOGD clast has TiO2 in excess of
5of22 B02204 Table 2. Gualala Formation Conglomerate Clast and ERP-GH-LQ Mafic Basement Terrane U/Pb Zircon Isotopic Age Data Concentrations Atomic Ratiosd Ages, Ma Amount 206 206 207 207 206 207 207 6/8-7/5 Analyzed, U, Pb*,c Common Pb Pb* Pb* Pb* Error, Pb* Pb* Pb* Correlation Sample Field Sample Propertiesa Fractionsb mg ppm ppm Pb, ppm 204Pb 238U %err 235U %err 206Pb % 238U 235U 206Pb Coefficient Jurassic Oceanic Gabbro and Diorite Clasts (JOGD): Shell Beach JOGD-5 RS92SP2 hbd qtz gabbro a D 64–100 1.98 114.0 2.517 0.047 1930 0.022609 0.19 0.15222 0.69 0.048832 0.66 144.1 143.9 139.8 0.2864 b N 100–150 2.25 117.0 2.606 0.037 2969 0.022691 0.47 0.15311 0.58 0.048936 0.33 144.6 144.6 144.8 0.8158 JOGD-8 RS92SP16 hbd qtz cpx gabbro
a N 64–100 1.95 99.17 2.152 0.461 305.3 0.022292 0.23 0.15521 0.98 0.050499 0.91 142.1 146.5 218.0 0.3896 BASIN GUALALA THE OF HISTORY CRETACEOUS AL.: ET SCHOTT b N < 150 6.12 199.7 4.312 0.070 2883 0.022148 0.26 0.14982 0.37 0.049060 0.27 141.2 141.8 150.7 0.6913 c N < 150 3.57 214.9 4.469 0.077 3505 0.021871 0.35 0.14912 0.35 0.049450 0.07 139.5 141.1 169.2 0.9811 d D < 150 2.77 201.9 4.347 0.060 4104 0.022668 0.49 0.15416 0.49 0.049326 0.06 144.5 145.6 163.4 0.9925 JOGD-9 RS92SP26 hbd qtz cpx gabbro a N < 100 1.51 221.4 5.729 0.030 7630 0.029040 0.19 0.16898 0.22 0.049213 0.12 158.6 158.5 158.0 0.8533
Jurassic Oceanic Gabbro and Diorite Clasts (JOGD): Sea Ranch Stable JOGD-14 RS92SP30 hbd qtz gabbro a D 64–100 2.37 292.6 6.136 0.059 5161 0.022055 0.75 0.15175 0.77 0.049904 0.19 140.6 143.5 190.5 0.9708 b M1 100–150 2.78 371.9 8.212 0.162 3102 0.022658 0.77 0.15243 0.78 0.048791 0.16 144.4 144.1 137.8 0.9804 c N 100–150 2.18 214.7 4.712 0.093 2884 0.022528 0.60 0.15186 0.61 0.048891 0.11 143.6 143.6 142.6 0.9840
Cretaceous Continental Granite and Rhyolite Clasts (KCGR): Shell Beach
6of22 KCGR-1 RS92SP15 hbd biot granodiorite a N < 64 1.40 884.3 13.457 0.054 6054 0.015188 0.20 0.12072 0.34 0.057651 0.27 97.2 115.7 516.5 0.6187 b D < 64 0.85 790.4 11.719 4.94 165.0 0.014825 0.25 0.10945 1.24 0.053546 1.18 94.9 105.5 352.0 0.3358 c N < 64 1.91 943.4 14.540 0.085 8080 0.015438 0.19 0.12144 0.22 0.057049 0.09 98.8 116.4 493.4 0.9094 d N 64–100 3.48 930.0 16.089 0.290 3424 0.017300 0.20 0.15114 0.23 0.063365 0.11 110.6 142.9 720.5 0.8894
Cretaceous Continental Granite and Rhyolite Clasts (KCGR): Sea Ranch Stable KCGR-4 RS92SP40 hbd biot granite a N < 64 0.83 961.3 14.848 0.068 2582 0.015795 0.20 0.10626 0.93 0.048791 0.90 101.0 102.5 137.8 0.2583 b N 64–100 1.87 1205 18.596 0.060 9479 0.015861 0.22 0.10604 0.32 0.048487 0.23 101.4 102.3 123.1 0.6961 c D 64–100 1.08 898.1 14.592 0.011 3450 0.016611 0.21 0.11596 0.79 0.050631 0.75 106.2 111.4 224.1 0.2986 d N 100–150 2.32 1283 19.391 0.057 18276 0.015590 0.23 0.10419 0.24 0.048469 0.06 99.7 100.6 122.2 0.9719
Cretaceous Continental Granite and Rhyolite Clasts (KCGR): Smuggler’s Cove KCGR-7 RS92SP45 biot granite a N 64–100 0.42 606.2 9.565 0.533 1054 0.015949 0.28 0.10983 0.32 0.049944 0.15 102.0 105.8 192.4 0.8923 b N < 64 0.69 976.2 15.459 0.261 3336 0.016013 0.21 0.10858 0.23 0.049178 0.09 102.4 104.7 156.3 0.9199 KCGR-8 RS92SP54 rhyolite a M2 64–100 1.09 566.7 11.668 1.24 610.0 0.020168 0.35 0.16874 0.38 0.060680 0.13 128.7 158.3 627.9 0.9386
Cretaceous Continental Granite and Rhyolite Clasts (KCGR): Black Point KCGR-9 RS92SP6 porphyritic biot granite a D 64–100 0.75 176.9 3.561 4.767 65.88 0.019911 0.30 0.13788 0.89 0.050222 0.80 127.1 131.2 205.3 0.4481 b N < 64 2.05 835.4 16.095 0.124 7335 0.019014 0.16 0.12871 0.17 0.049093 0.06 121.4 122.9 152.3 0.9442 c N 100–150 0.5 726.0 13.893 0.653 1264 0.019100 0.21 0.12813 0.25 0.048651 0.14 122.0 122.4 131.1 0.8480 d N 100–150 AB 0.31 322.7 6.332 0.599 600.7 0.019603 0.48 0.13025 0.54 0.048188 0.25 125.1 124.3 108.5 0.8880
e N < 64 1.07 939.4 18.418 0.080 10764 0.019320 5.52 0.13103 5.52 0.049189 0.21 123.4 125.0 156.8 0.9992 B02204 KCGR-11 RS92SP8 porphyritic biot granite a M2 64–100 2.37 612.6 9.650 1.408 455.0 0.015664 0.17 0.10774 0.63 0.049885 0.57 100.2 103.9 189.6 0.4661 b N 64–100 1.5 335.5 5.643 0.835 442.0 0.016681 0.63 0.12164 0.65 0.052888 0.14 106.6 116.6 324.0 0.9765 B02204
Table 2. (continued) Concentrations Atomic Ratiosd Ages, Ma Amount 206 206 207 207 206 207 207 6/8-7/5 Analyzed, U, Pb*,c Common Pb Pb* Pb* Pb* Error, Pb* Pb* Pb* Correlation Sample Field Sample Propertiesa Fractionsb mg ppm ppm Pb, ppm 204Pb 238U %err 235U %err 206Pb % 238U 235U 206Pb Coefficient c N 100–150 0.1 325.1 5.626 0.478 479.0 0.017210 1.58 0.12225 1.69 0.051518 0.58 110.0 117.1 264.1 0.9387 d N 100–150 AB 0.14 633.5 13.733 0.429 1370 0.021418 0.52 0.17264 0.55 0.058459 0.18 136.6 161.7 547.0 0.9459 KCGR-13 RS92SP55 coarse-grained, equigranular biot granite BASIN GUALALA THE OF HISTORY CRETACEOUS AL.: ET SCHOTT a M2 64–100 0.56 1366 19.361 0.661 1799 0.014388 0.20 0.09543 0.23 0.048106 0.11 92.1 92.6 104.5 0.8874 b N 64–100 0.62 1041 15.685 0.425 2185 0.015247 0.21 0.10173 0.22 0.048393 0.08 97.5 98.4 118.5 0.9352 KCGR-14 RS92SP56 coarse-grained, equigranular biot granite a N 64–100 0.18 1749 27.732 0.455 2865 0.015717 0.27 0.10447 0.32 0.048208 0.16 100.5 100.9 109.5 0.8587 KCGR-15 RS92SP58 porphyritic biot granite a N < 100 3.06 756.1 12.809 1.608 528.1 0.016760 0.18 0.11232 0.22 0.048604 0.13 107.1 108.1 128.8 0.8187 b N > 100 1.40 701.5 12.345 0.555 1360 0.017524 0.28 0.12401 0.46 0.051326 0.35 112.0 118.7 255.5 0.6519 KCGR-17 RS92SP62 porphyritic biot granite a N > 100 2.67 623.2 9.394 0.306 1915 0.014895 0.19 0.09870 0.26 0.048071 0.18 95.3 95.6 102.7 0.7396 KCGR-18 RS92SP70 coarse-grained, equigranular biot granite a N < 64 5.12 1305 19.603 0.463 2738 0.015326 0.38 0.10276 0.41 0.048632 0.15 98.0 99.3 130.1 0.9297 7of22 b N 64–100 4.36 1239 16.551 0.516 2080 0.013636 0.60 0.09026 0.63 0.048009 0.17 87.3 87.7 99.7 0.9645
Jurassic Mafic Basement Terrane (ERP-GH-LQ): Eagle Rest Peak (ERP) ERP-11 RS92ERP15 hbd qtz gabbro a D > 150 2.84 169.5 4.327 0.041 4777 0.025040 0.23 0.17050 0.29 0.049384 0.18 159.4 159.9 166.1 0.7909 b N > 150 6.36 208.3 5.297 0.036 7781 0.024784 0.29 0.16832 0.31 0.049257 0.11 157.8 158.0 160.1 0.9373 c N > 150 AB 3.15 343.9 8.427 0.266 1351 0.023769 0.47 0.16211 0.50 0.049464 0.17 151.4 152.5 169.9 0.9418 ERP-14 RS92SC7 hbd tonalite a N 100–150 1.86 215.1 5.18 0.012 8810 0.024656 0.19 0.16835 0.34 0.049519 0.27 157.0 158.0 172.5 0.5891
Jurassic Mafic Basement Terrane (ERP-GH-LQ): Gold Hill (GH) GH-11 RS92CV6 hbd qtz gabbro a N > 100 0.84 499.9 12.496 0.061 8231 0.025085 0.28 0.17036 0.33 0.049256 0.19 159.7 159.7 160.0 0.8309
Jurassic Mafic Basement Terrane (ERP-GH-LQ): Logan Quarry (LQ) LQ-8 RS92CH1 hbd qtz gabbro a D 100–150 2.75 114.8 2.727 0.024 4307 0.024600 0.19 0.16309 0.84 0.048085 0.78 156.7 153.4 103.4 0.4184 b N > 150 7.01 71.09 1.685 0.043 2365 0.024724 0.20 0.16829 0.24 0.049368 0.13 157.4 157.9 165.3 0.8431 c N > 150 AB 6.75 67.36 1.630 0.087 961.6 0.025079 0.24 0.16888 0.77 0.048840 0.73 159.7 158.4 140.1 0.3231 aAbbreviations are are biot, biotite; cpx, clinopyroxene; hbd, hornblende; qtz, quartz. bZircon fractions separated by size and magnetic properties. All magnetic separations performed on a Frantz Magnetic Barrier Separator, model LB-1, with current of 1.7 A and a forward slope of 17.5 . Magnetic fractions as follows: D, diamagnetic; N, nonmagnetic at 2 side slope (weakly paramagnetic); M number is paramagnetic at number of degrees of side slope. Size fraction cuts at 64, 100, and 150 mm. Dissolution and chemical separation techniques modified from Krogh [1973]. Data reduction and error analysis based on Ludwig [1989]. Decay constants used for age calculation are l238 U = 1.55125 10 10 yr 1 and l235 U= 9.8485 10 10 yr 1 [Steiger and Ja¨ger, 1977]. cRadiogenic; nonradiogenic correction based on measured Pb blanks of 20 to 380 pgr (30 pg average) (1:19.00:15.35:37.62) and initial Pb from feldspar or from Stacey and Kramers [1975] model. d Corrected for mass fractionation of 0.1% per amu for Pb analyses and 0.12% per amu for U analyses based on replicate analyses of NBS-981, NBS-982, NBS-983, and U500 standards. B02204 B02204 SCHOTT ET AL.: CRETACEOUS HISTORY OF THE GUALALA BASIN B02204
Table 3a. Gualala Formation Conglomerate Clasts and Jurassic Mafic Basement Terrane (ERP-GH-LQ) Major Element Chemistrya
Sample Field Sample SiO2 Al2O3 Fe2O3* MgO CaO Na2OK2OTiO2 P2O5 MnO LOI Total Jurassic Oceanic Gabbro and Diorite Clasts (JOGD) JOGD-1 CJ-826-1A 55.2 15.36 8.91 5.43 7.54 3.92 0.14 1.18 0.15 0.13 1.31 99.27 JOGD-2 CJ-826-1C 48.9 20.13 7.87 5.51 12.30 2.52 0.19 0.70 0.05 0.13 1.10 99.40 JOGD-3 RS92GU5 JOGD-4 RS96GU2 JOGD-5 RS92SP2 60.13 16.67 6.22 2.86 6.63 3.49 0.40 0.55 0.12 0.14 1.82 99.03 JOGD-6 RS92SP13 75.46 13.26 0.74 0.22 2.02 3.97 2.29 0.08 0.02 0.03 0.97 99.06 JOGD-7 RS92SP14 47.21 19.94 5.04 9.50 14.29 1.73 0.07 0.17 0.01 0.08 1.37 99.41 JOGD-8 RS92SP16 57.18 15.85 7.27 4.42 7.16 3.59 0.93 0.52 0.09 0.21 1.64 98.86 JOGD-9 RS92SP26 53.14 16.58 10.06 5.53 8.88 2.12 0.36 0.48 0.04 0.18 2.44 99.81 JOGD-10 CJ-829-2A 47.6 20.70 4.73 8.37 14.83 1.80 0.20 0.16 0.01 0.08 1.47 99.95 JOGD-11 CJ-829-2D 58.12 16.58 8.12 3.34 6.89 3.05 1.19 0.53 0.12 0.14 2.30 100.38 JOGD-12 CJ-829-3A 47.02 17.67 5.49 10.75 14.68 1.53 0.12 0.21 0.01 0.09 1.27 98.84 JOGD-13 RS92SP27 58.09 17.87 6.19 3.34 7.90 2.53 0.26 0.34 0.06 0.14 2.75 99.47 JOGD-14 RS92SP30 60.72 17.23 5.50 2.74 6.32 3.32 0.86 0.35 0.10 0.11 1.87 99.12 JOGD-15 RS92SP31 47.63 19.56 6.10 7.76 14.44 1.90 0.20 0.42 0.04 0.10 1.40 99.55 JOGD-16 RS92SP32 62.66 16.11 5.90 2.28 6.15 2.71 1.00 0.27 0.09 0.14 2.10 99.41
Cretaceous Continental Granite and Rhyolite Clasts (KCGR) KCGR-1 RS92SP15 68.09 15.46 3.10 1.05 2.81 3.97 3.04 0.51 0.13 0.04 0.83 99.03 KCGR-2 CJ-829-3A KCGR-3 CJ-829-4B 70.39 12.74 2.08 0.48 3.85 3.57 3.35 0.24 0.05 0.05 2.30 99.10 KCGR-4 RS92SP40 73.89 13.22 1.44 0.41 2.12 3.62 3.79 0.19 0.04 0.03 1.16 99.91 KCGR-5 RS92SP42 74.73 13.38 1.51 0.40 0.73 4.16 3.49 0.23 0.05 0.03 0.85 99.56 KCGR-6 RS92SP43 76.81 12.53 0.50 0.11 0.35 4.09 4.11 0.04 0.01 0.02 0.45 99.02 KCGR-7 RS92SP45 74.73 13.22 1.81 0.26 0.68 4.20 3.80 0.14 0.03 0.03 1.55 100.45 KCGR-8 RS92SP54 75.47 13.24 1.80 0.40 <0.01 3.92 3.57 0.26 0.04 0.03 1.03 99.76 KCGR-9 RS92SP6 76.58 12.80 1.46 0.20 0.87 3.23 3.80 0.15 0.02 0.02 0.56 99.69 KCGR-10 RS92SP7 77.94 12.13 0.88 0.12 0.39 3.04 5.31 0.06 0.01 0.01 0.33 100.22 KCGR-11 RS92SP8 76.61 12.57 1.52 0.19 0.34 3.28 4.22 0.15 0.01 0.01 0.72 99.62 KCGR-12 RS92SP10 72.44 14.19 2.50 1.03 0.78 3.64 3.57 0.48 0.10 0.08 1.36 100.17 KCGR-13 RS92SP55 78.61 11.91 0.76 0.16 0.30 4.23 3.79 0.07 0.02 0.01 0.40 100.26 KCGR-14 RS92SP56 78.16 12.09 0.84 0.12 0.10 3.95 3.72 0.09 0.00 0.00 0.54 99.61 KCGR-15 RS92SP58 78.01 11.74 1.07 0.24 0.07 2.96 4.66 0.13 0.02 0.02 0.74 99.66 KCGR-16 RS92SP60 79.70 11.67 1.09 0.13 0.08 3.73 3.42 0.09 0.00 0.01 0.45 100.37 KCGR-17 RS92SP62 76.62 11.88 1.47 0.44 0.21 3.72 4.66 0.22 0.04 0.02 0.73 100.00 KCGR-18 RS92SP70 78.42 11.65 1.16 0.12 0.29 3.84 4.31 0.08 0.02 0.02 0.50 100.40
Eagle Rest Peak (ERP) ERP-1 RS92ER1 49.03 14.99 10.63 7.48 12.32 2.30 0.18 1.45 0.10 0.16 0.87 99.51 ERP-2 CJ-40A 50.64 10.19 5.70 15.59 15.43 0.79 0.11 0.12 0.00 0.14 0.91 99.62 ERP-3 CJ-41A 49.88 14.37 2.54 10.70 21.12 0.64 0.04 0.30 0.01 0.03 0.51 100.14 ERP-4 CJ-41A-A 47.28 17.17 6.41 10.85 14.03 1.14 0.28 0.24 0.01 0.09 1.48 98.98 ERP-5 CJ-42A 48.18 17.17 5.29 10.92 15.45 1.46 0.04 0.21 0.01 0.09 1.69 100.51 ERP-6 CJ-42B ERP-7 CJ-43A 69.45 14.00 3.36 0.84 2.34 4.25 3.02 0.34 0.07 0.06 1.91 99.64 ERP-8 CJ-43C 56.16 14.59 6.55 6.99 9.99 2.48 0.59 0.40 0.09 0.15 1.14 99.13 ERP-9 RS92ER2 44.4 21.32 4.74 9.86 15.98 0.48 0.03 0.05 0.01 0.08 1.59 98.54 ERP-10 RS92ER9 49.7 17.26 4.83 8.55 14.77 2.31 0.10 0.30 0.01 0.09 1.69 99.61 ERP-11 RS92ER15 55.07 16.45 6.78 6.35 9.52 2.53 0.23 0.35 0.03 0.12 1.35 98.78 ERP-12 RS92ER18 52.83 1.91 8.56 23.70 12.56 0.61 0.05 0.09 <0.01 0.22 0.45 100.98 ERP-13 RS92ER28 47.63 18.01 5.34 10.40 14.83 1.53 0.06 0.21 0.01 0.09 1.56 99.67 ERP-14 RS92SC1 63.9 15.79 6.01 2.03 5.79 2.78 0.65 0.30 0.05 0.13 1.39 98.82 ERP-15 RS92SC7 63.1 16.06 6.38 2.14 6.10 2.82 0.68 0.33 0.06 0.13 1.33 99.13 ERP-16 CJ-40B ERP-17 CJ-42BQ
Gold Hill (GH) GH-1 CJ-21 GH-2 CJ-22 GH-3 CJ-24 52.31 14.32 11.90 7.39 10.57 1.39 0.22 0.58 0.08 0.19 0.74 99.69 GH-4 CJ-25 GH-5 CJ-27 GH-6 CJ-28 GH-7 CJ-29 GH-8 SH-4 51.65 17.85 9.58 5.95 11.10 1.49 0.17 0.31 0.03 0.16 0.36 98.65 GH-9 CJ-GH-1 GH-10 RS92CV5 52.29 15.45 11.31 6.36 9.84 1.47 0.22 0.58 0.15 0.19 2.46 100.32 GH-11 RS92CV6 52.25 14.73 11.49 6.91 9.98 1.25 0.20 0.56 0.05 0.21 2.36 99.99 GH-12 CJ-24V GH-13 CJ-25VA GH-14 CJ-25VB
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Table 3a. (continued)
Sample Field Sample SiO2 Al2O3 Fe2O3* MgO CaO Na2OK2OTiO2 P2O5 MnO LOI Total Logan Quarry (LQ) LQ-1 CJ-IUF LQ-2 CJ-2 LQ-3 CJ-LQ-1 47.31 24.45 2.83 5.76 15.34 1.81 0.03 0.13 0.01 0.05 1.73 99.45 LQ-4 CJ-IF 52.11 18.17 9.76 4.42 9.89 2.03 0.20 0.61 0.08 0.16 1.32 98.75 LQ-5 CJ-3 LQ-6 SH-6 LQ-7 SJB-1 LQ-8 RS92CH1 56.57 18.35 7.38 3.01 7.88 2.90 0.57 0.53 0.08 0.13 1.94 99.34 LQ-9 RS92SJ1 48.37 15.25 7.70 10.37 13.29 1.79 0.07 0.66 <0.01 0.12 1.11 98.73 a Major element oxides in weight percent. LOI, loss on ignition. All Fe is Fe2O3.
1 wt %, which plots in the low-Ti end of the Franciscan mid- Ross, 1990; Mattinson, 1990]. The eNd(t) values range from ocean ridge basalt (MORB) field [Shervais, 1990]. +1 to 7, also indicating involvement of old continental crust, [13] The KCGR clasts are uniformly enriched in light rare and are most similar to those of the central and eastern parts of earth elements (LREE; CeN 50–120 times chondritic) the California batholiths [DePaolo, 1981; Mattinson, 1990]. relative to heavy rare earth elements (HREE; YbN 5– Sr-Nd isotope variations of the bulk of the KCGR clasts 30 times chondritic), and have pronounced negative Eu diverge significantly from the tight trend characteristic of anomalies (Figure 7a). These patterns are typical of granites Cretaceous Cordilleran granitoids (Figure 9c), a feature that in the southern Sierra Nevada and central Salinian block seems to be restricted to the Mojave-Salinia segment of the arc [Ross, 1982, 1989]. A single exception is the REE pattern [Schott and Johnson, 1998a]. Lead isotope variations (Figure for clast KCGR-6, which has high total REE concentrations 10 and Table 4) of KCGR clasts follow the trend established ( 30–40 times chondritic) but a relatively low LREE/ by other Cretaceous granitoids in the California batholiths HREE ratio, possibly indicative of fractionation of LREE- [Mattinson, 1990; Chen and Tilton, 1991]. The KCGR clasts enriched accessory minerals [Miller and Mittlefehldt, 1982]. have moderately high 207Pb/204Pb ratios (Figure 10), similar [14] The REE patterns of the JOGD clasts can be sub- to those seen in the Salinian block and western Mojave divided into three subgroups (Figure 7b). The most mafic [Wooden et al., 1988; Mattinson, 1990] but not at all similar clasts, which have no modal quartz and display cumulate to the extremely high 207Pb/204Pb ratios that are characteristic textures, have low LREE/HREE ratios, low overall REE of the eastern Mojave Desert [Wooden et al., 1988]. concentrations, and moderate to strongly positive Eu [17] Measured radiogenic and stable isotope composi- anomalies. These samples likely reflect plagioclase accu- tions indicate that the sources of the JOGD clasts are much mulation. The majority of JOGD clasts have relatively flat more primitive than those of the KCGR clasts and/or REE patterns ( 6–11 times chondritic) and little or no Eu interacted less with old continental crust. Initial 87Sr/86Sr anomaly. Finally, four of the more silicic JOGD clasts have isotope ratios of JOGD clasts are generally between 0.7030 somewhat more fractionated REE patterns that are charac- and 0.7040 (Table 4), relatively primitive compositions that terized by high LREE/HREE ratios and small to moderate are indicative of a depleted-mantle source, although a few negative Eu anomalies. are as high as 0.7052; these high ratios seem likely to reflect 3.1.3. Isotopic Compositions alteration by seawater [e.g., McCulloch et al., 1981], in 18 [15] Oxygen isotope ratios of the KCGR clasts (d Omagma concert with their high dD values (Figure 8). The eNd(t) =+8.2to+11.2%, as inferred from analyses of quartz values range from +5.5 to +8.6 and also indicate a depleted- separates; Table 4) are higher than those of the mantle and mantle source. For the most part, Sr and Nd isotope require a significant component of upper continental crustal variations (Figure 9c) cluster within the fields of primitive 18 material. In the California batholiths, d Omagma values arcs or EMORB [e.g., Zindler et al., 1979; Hart et al., 1986; greater than +9% are common only in the eastern Peninsular Lin et al., 1990; Stern et al., 1993]. Lead isotope compo- Ranges, the Salinian block, the western Mojave Desert, and sitions are more radiogenic than those of modern east the southeastern SNB [Taylor and Silver, 1978; Masi et al., Pacific MORB from the Juan de Fuca plate and are more 1981; Solomon, 1989; Solomon and Taylor, 1989]. Distinctly primitive than those of Coast Range Ophiolite samples from 18 lower d Omagma values of JOGD clasts (+6.3% to +9.0%) the Point Sal Ophiolite, but similar to those of the Josephine probably reflect mantle-derived magmas that experienced Ophiolite [Chen and Shaw, 1982] (Figure 10). A relatively limited interaction with the crust. dD values of two JOGD steep linear trend in both 206Pb/204Pb-207Pb/204Pb and clasts are relatively high (about 50%), suggesting hydro- 206Pb/204Pb-208Pb/204Pb variations (Figure 10) in some of thermal alteration with ocean water (Figure 8). the Gualala gabbros may reflect assimilation and mixing [16] Strontium, Nd, and Pb isotope compositions of the with an older Pb component that had a low Th/U ratio KCGR clasts indicate involvement of old continental crust in [Church, 1976; Woodhead and Fraser, 1985]. source bodies (Figure 9). Initial 87Sr/86Sr isotope ratios of KCGR clasts are uniformly greater than 0.7060 and most (12 3.2. Mafic Basement Terrane (ERP-LQ-GH) 87 86 out of 16) clasts have Sr/ Srinit between 0.7075 and 0.7090 [18] It is generally accepted that Jurassic gabbroic rocks (Table 4). These are relatively radiogenic Sr isotope compo- at Eagle Rest Peak (ERP), Logan Quarry (LQ), and Gold sitions for the Cretaceous batholiths of California, and such Hill (GH) were derived from the same mafic basement compositions are diagnostic of the central or eastern regions terrane [Ross et al., 1973; James et al., 1993], and the ERP of the arc [Kistler and Peterman, 1973, 1978; Kistler and locality probably represents exposures of this terrane that
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Table 3b. Gualala Formation Conglomerate Clasts and Jurassic Mafic Basement Terrane (ERP-GH-LQ) Trace Element Chemistrya Sample Field Sample Rb Sr Y Zr Nb Ba Sc V Cr Ni Cu Zn Jurassic Oceanic Gabbro and Diorite Clasts (JOGD) JOGD-1 CJ-826-1A JOGD-2 CJ-826-1C JOGD-3 RS92GU5 JOGD-4 RS96GU2 JOGD-5 RS92SP2 10 351 23 71 8 138 26 236 15 7 14 76 JOGD-6 RS92SP13 33 206 13 52 2 984 4 7 2 3 <1 10 JOGD-7 RS92SP14 7 158 10 18 5 19 32 85 569 150 69 39 JOGD-8 RS92SP16 20 386 39 65 9 223 39 175 75 17 59 90 JOGD-9 RS92SP26 12 195 20 31 6 145 60 362 59 16 23 89 JOGD-10 CJ-829-2A JOGD-11 CJ-829-2D JOGD-12 CJ-829-3A JOGD-13 RS92SP27 9 311 16 41 5 97 27 208 14 2 7 52 JOGD-14 RS92SP30 23 364 22 107 8 315 18 97 40 16 1 50 JOGD-15 RS92SP31 10 222 15 32 6 71 45 165 86 81 ND 22 JOGD-16 RS92SP32 19 345 17 52 6 313 21 132 11 1 3 41
Cretaceous Continental Granite and Rhyolite Clasts (KCGR) KCGR-1 RS92SP15 94 490 13 199 11 940 7 35 8 7 ND 63 KCGR-2 CJ-829-3A KCGR-3 CJ-829-4B KCGR-4 RS92SP40 97 151 21 139 10 1377 5 10 6 6 ND 28 KCGR-5 RS92SP42 113 196 29 193 13 874 8 7 2 7 ND 38 KCGR-6 RS92SP43 200 13 85 95 25 158 10 ND 2 8 ND 17 KCGR-7 RS92SP45 138 107 22 181 12 894 8 ND 2 7 ND 42 KCGR-8 RS92SP54 103 113 45 191 11 928 9 18 0 1 3 79 KCGR-9 RS92SP6 87 178 15 83 5 653 4 14 4 1 5 16 KCGR-10 RS92SP7 243 86 8 72 ND 95 2 ND 3 6 ND ND KCGR-11 RS92SP8 142 132 21 127 8 725 3 9 2 1 5 29 KCGR-12 RS92SP10 123 289 20 178 15 738 8 47 11 7 4 65 KCGR-13 RS92SP55 149 24 12 162 12 470 3 ND 2 8 ND 32 KCGR-14 RS92SP56 126 42 18 170 17 627 3 4 1 1 15 28 KCGR-15 RS92SP58 156 131 9 95 16 502 3 10 8 1 <1 10 KCGR-16 RS92SP60 115 52 35 193 16 667 5 5 3 1 2 28 KCGR-17 RS92SP62 190 161 12 124 <1 669 4 20 7 11 <1 10 KCGR-18 RS92SP70 193 39 8 144 11 480 3 3 2 12 <1 117
Eagle Rest Peak (ERP) ERP-1 RS92ER1 2 160 30 89 4 29 52 271 340 89 65 78 ERP-2 CJ-40A ERP-3 CJ-41A ERP-4 CJ-41A-A ERP-5 CJ-42A ERP-6 CJ-42B ERP-7 CJ-43A ERP-8 CJ-43C ERP-9 RS92ER2 6 133 8 14 6 ND 29 54 302 73 0 16 ERP-10 RS92ER9 7 180 12 19 5 7 58 153 390 105 59 37 ERP-11 RS92ER15 9 177 25 77 7 90 49 159 11 17 148 71 ERP-12 RS92ER18 1 18 2 <1 1 12 84 175 1900 138 8 47 ERP-13 RS92ER28 7 160 11 18 4 2 45 112 859 184 97 46 ERP-14 RS92SC1 16 141 20 32 7 198 31 123 11 3 1559 373 ERP-15 RS92SC7 15 158 18 39 7 186 30 134 10 3 26 66 ERP-16 CJ-40B ERP-17 CJ-42BQ
Gold Hill (GH) GH-1 CJ-21 GH-2 CJ-22 GH-3 CJ-24 GH-4 CJ-25 GH-5 CJ-27 GH-6 CJ-28 GH-7 CJ-29 GH-8 SH-4 GH-9 CJ-GH-1 GH-10 RS92CV5 3 106 13 30 2 35 73 377 219 31 104 82 GH-11 RS92CV6 3 106 11 26 2 19 77 405 235 33 123 87 GH-12 CJ-24V GH-13 CJ-25VA GH-14 CJ-25VB
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Table 3b. (continued) Sample Field Sample Rb Sr Y Zr Nb Ba Sc V Cr Ni Cu Zn Logan Quarry (LQ) LQ-1 CJ-IUF LQ-2 CJ-2 LQ-3 CJ-LQ-1 LQ-4 CJ-IF LQ-5 CJ-3 LQ-6 SH-6 LQ-7 SJB-1 LQ-8 RS92CH1 12 213 20 39 7 162 30 185 14 3 44 60 LQ-9 RS92SJ1 1 135 16 23 2 <6 72 210 757 203 6 25 Sample Field Sample Ce Nd Sm Eu Gd Dy Er Yb Jurassic Oceanic Gabbro and Diorite Clasts (JOGD) JOGD-1 CJ-826-1A 12.535 9.950 3.006 1.093 3.759 4.464 2.801 2.623 JOGD-2 CJ-826-1C 5.154 4.158 1.321 0.584 1.714 2.086 1.320 1.242 JOGD-3 RS92GU5 8.172 6.371 2.001 0.721 2.606 3.065 1.936 1.855 JOGD-4 RS96GU2 3.909 3.305 1.139 0.422 1.593 2.066 1.440 1.538 JOGD-5 RS92SP2 12.097 8.584 2.244 0.793 2.634 2.884 1.914 1.977 JOGD-6 RS92SP13 23.061 9.685 1.767 0.438 1.585 1.706 1.220 1.543 JOGD-7 RS92SP14 0.864 0.931 0.362 0.309 0.570 0.686 0.414 0.369 JOGD-8 RS92SP16 20.764 18.110 4.920 1.103 5.090 5.603 3.785 4.178 JOGD-9 RS92SP26 4.807 4.030 1.371 0.536 1.919 2.347 1.598 1.633 JOGD-10 CJ-829-2A 1.037 0.812 0.299 0.286 0.443 0.558 0.282 0.294 JOGD-11 CJ-829-2D 21.851 12.969 2.953 0.778 2.812 2.928 1.854 1.896 JOGD-12 CJ-829-3A JOGD-13 RS92SP27 4.834 3.587 1.024 0.501 1.329 1.553 1.090 1.212 JOGD-14 RS92SP30 22.547 11.600 2.490 0.785 2.539 2.597 1.732 1.869 JOGD-15 RS92SP31 3.280 2.805 0.940 0.431 1.345 1.583 1.008 0.928 JOGD-16 RS92SP32 7.298 4.735 1.325 0.467 1.570 1.785 1.277 1.496
Cretaceous Continental Granite and Rhyolite Clasts (KCGR) KCGR-1 RS92SP15 63.868 26.131 4.450 0.938 3.161 2.178 1.047 0.983 KCGR-2 CJ-829-3A 52.143 25.136 5.535 1.197 5.285 5.625 3.524 3.617 KCGR-3 CJ-829-4B 66.767 27.759 4.987 0.820 3.994 3.409 1.873 1.761 KCGR-4 RS92SP40 49.658 21.356 4.104 0.717 3.542 3.263 1.838 1.720 KCGR-5 RS92SP42 57.719 24.645 5.089 0.725 4.409 4.624 2.748 2.892 KCGR-6 RS92SP43 24.469 18.239 7.878 0.273 9.790 14.057 8.563 7.964 KCGR-7 RS92SP45 58.835 25.316 4.938 0.734 3.961 3.957 2.356 2.636 KCGR-8 RS92SP54 61.688 33.110 7.039 1.008 6.608 7.022 4.322 4.221 KCGR-9 RS92SP6 32.566 13.183 2.586 0.435 2.410 2.514 1.669 1.883 KCGR-10 RS92SP7 49.405 13.318 2.128 0.135 1.704 1.645 1.182 1.767 KCGR-11 RS92SP8 48.719 19.444 3.489 0.584 3.077 3.472 2.303 2.438 KCGR-12 RS92SP10 67.741 25.671 4.592 1.067 3.690 3.445 2.066 2.291 KCGR-13 RS92SP55 49.237 18.428 2.897 0.381 2.025 2.031 1.280 1.441 KCGR-14 RS92SP56 58.677 25.770 4.735 0.569 3.561 3.390 2.088 0.941 KCGR-15 RS92SP58 52.664 15.454 2.344 0.293 1.672 1.683 1.101 1.442 KCGR-16 RS92SP60 45.695 21.229 5.023 0.628 5.086 6.139 4.059 4.323 KCGR-17 RS92SP62 53.692 17.716 2.801 0.308 1.986 1.767 1.087 1.289 KCGR-18 RS92SP70 46.306 17.253 3.040 0.498 2.125 1.712 0.964 1.080
Eagle Rest Peak (ERP) ERP-1 RS92ER1 10.387 9.089 3.033 1.168 4.120 4.779 2.947 2.750 ERP-2 CJ-40A 0.558 0.657 0.283 0.187 0.430 0.571 0.367 0.337 ERP-3 CJ-41A 1.339 1.859 0.785 0.466 1.165 1.469 0.879 0.777 ERP-4 CJ-41A-A 1.297 1.337 0.536 0.356 0.809 1.017 0.633 0.551 ERP-5 CJ-42A 0.734 0.978 0.463 0.303 0.732 0.970 0.591 0.530 ERP-6 CJ-42B ERP-7 CJ-43A 0.789 0.998 0.445 0.301 0.703 0.912 0.553 0.491 ERP-8 CJ-43C 10.922 6.805 1.764 0.600 1.894 2.005 1.277 1.307 ERP-9 RS92ER2 ERP-10 RS92ER9 ERP-11 RS92ER15 ERP-12 RS92ER18 ERP-13 RS92ER28 ERP-14 RS92SC1 ERP-15 RS92SC7 5.655 3.856 1.137 0.458 1.541 1.922 1.400 1.562 ERP-16 CJ-40B ERP-17 CJ-42BQ
Gold Hill (GH) GH-1 CJ-21 GH-2 CJ-22 GH-3 CJ-24 4.888 4.342 1.394 0.479 1.817 2.298 1.559 1.603
11 of 22 B02204 SCHOTT ET AL.: CRETACEOUS HISTORY OF THE GUALALA BASIN B02204
Table 3b. (continued) Sample Field Sample Rb Sr Y Zr Nb Ba Sc V Cr Ni Cu Zn GH-4 CJ-25 GH-5 CJ-27 GH-6 CJ-28 GH-7 CJ-29 GH-8 SH-4 2.771 2.279 0.752 0.386 1.006 1.326 0.923 0.991 GH-9 CJ-GH-1 GH-10 RS92CV5 GH-11 RS92CV6 GH-12 CJ-24V GH-13 CJ-25VA GH-14 CJ-25VB
Logan Quarry (LQ) LQ-1 CJ-IUF LQ-2 CJ-2 LQ-3 CJ-LQ-1 0.679 n.d. 0.346 0.230 0.532 0.471 0.282 0.248 LQ-4 CJ-IF 4.834 4.911 1.635 0.636 2.190 2.722 1.828 1.774 LQ-5 CJ-3 LQ-6 SH-6 LQ-7 SJB-1 LQ-8 RS92CH1 LQ-9 RS92SJ1 aTrace element abundances are in parts per million; ND, not determined. All abundances are determined by XRF, except Rb, Sr, and REE are determined by isotope dilution mass spectrometry. have not been displaced by Neogene tectonics. U/Pb zircon relatively distinctive constraints on sediment sources. The crystallization ages in all localities are concordant or near data provide general constraints on source terranes, rather concordant at about 158 Ma to 160 Ma (Figure 4b and than identifying locally restrictive sources, such as those Table 2). These U/Pb ages are in excellent agreement with required to establish piercing points across the San Andreas previously published ages from these localities by James et fault system [Cowan et al., 1997]. Nevertheless, provenance al. [1993]. Particularly important is the 160 ± 1 Ma constraints have important implications for Late Cretaceous concordant zircon fraction from Gold Hill, a locality which tectonics and paleolatitudes of the coastal components of had previously yielded a discordant U/Pb zircon age of the western Cordillera. 144 Ma [James et al., 1993]. We therefore propose an emplacement age of 160 ± 2 Ma for all three bodies. 4.1. Provenance Constraints: Juxtaposed Oceanic and Chemical and isotopic compositions of ERP-LQ-GH are Continental Sources in a Near-Forearc Setting broadly similar to those of the JOGD suite (Tables 3 and 4). [20] The presence of cobble- and boulder-sized con- Reitz [1986] recognized a boninitic crystallization sequence glomerate clasts of markedly different compositions within at ERP, a sequence that is commonly associated with the same beds in the Gualala Formation requires that the magmatism in a forearc tectonic environment [Hawkins et terranes from which these two distinct rock types were al., 1984]. An oceanic arc origin is also indicated by the derived were juxtaposed adjacent to the basin during the lack of TiO2 enrichment with increasing FeO*/MgO ratios latest Cretaceous. In the Cordilleran batholiths of western (Figure 6) [Reitz, 1986]. The REE patterns from ERP-LQ- North America, rocks of these contrasting types are not GH are generally flat, although some of the cumulate rocks known to occur in close proximity to each other; rather, from Eagle Rest Peak have strongly LREE depleted pat- they are most often separated by a wide expanse of rocks terns, and one quartz-bearing gabbro has a weakly LREE- of intermediate compositions (i.e., tonalite and granodio- enriched pattern (Figure 7c); in general, however, these rite) that have a continuous gradation in isotopic compo- patterns overlap those of the JOGD clasts. Whole rock d18O sitions [e.g., Kistler and Peterman, 1973, 1978; Taylor and and dD values of the ERP-LQ-GH rocks (Figure 8 and Silver, 1978; DePaolo, 1981; Chen and Tilton, 1991; Table 4) scatter from primary igneous compositions; the Bateman, 1992]. Although there are minor to significant relatively high dD values most likely reflect hydrothermal isotopic steps in some regions, such as the central Penin- alteration by seawater, similar to the JOGD clasts. Rocks sular Ranges batholith [Silver et al., 1979] or the western from ERP-LQ-GH have initial 87Sr/86Sr ratios that are Idaho batholith [Fleck and Criss, 1985], granites and mostly in the range of 0.7025 to 0.7035, and are therefore rhyolites of continental affinity are not found in close slightly more primitive than most of the JOGD suite, proximity to the forearc in any of the California batholiths although some overlap occurs (Figure 9). The eNd(t) value when viewed in the configuration of their magmatic ranges from +5.1 to +8.7, in close agreement with values emplacement. observed for the JOGD clasts (Figure 9). [21] Of the exposed possible source terranes, it seems most likely that the Gualala Formation conglomerates were derived from the Gabilan-San Emigdio Ranges 4. Discussion region that would have been a contiguous source terrane [19] Provenance characteristics of Gualala Formation in the Late Cretaceous (Figure 11). In the San Emigdio conglomerates, taken in their depositional context, provide Mountains of the southern Sierran tail, granitic rocks of
12 of 22 B02204 Table 4. Gualala Formation Conglomerate Clasts and Jurassic Mafic Basement Terrane (ERP-GH-LQ) Isotope Dataa
87 87 87 147 143 206 207 208 18 Field Age, Rb, Sr, Rb Sr Sr Sm, Nd, Sm Nd Pb Pb Pb d O 86 Measured 86 Initial 86 144 Measured 144 204 204 204 Sample Sample Ma ppm ppm Sr Sr Sr ppm ppm Nd Nd Nd (t) Pb Pb Pb Quartz Magma WR dDWR Jurassic Oceanic Gabbros and Quartz Diorites (JOGD) JOGD-1 CJ-826-1A 150 2.394 200.5 0.0345 0.703814 0.70374 3.006 9.950 0.1824 0.513058 8.47 JOGD-2 CJ-826-1C 150 2.845 215.1 0.0383 0.703081 0.70300 1.321 4.179 0.1909 0.513050 8.15 JOGD-3 RS92GU5 150 1.995 6.332 0.1903 0.513071 8.58 JOGD-4 RS96GU2 150 1.135 3.285 0.2087 0.513073 8.25 JOGD-5 RS92SP2 145 6.517 343.6 0.0549 0.7033918 0.70328 2.244 8.555 0.1584 0.513012 8.00 18.292 15.520 38.825 9.0 7.5 JOGD-6 RS92SP13 150 33.000 206.0 0.4315 1.761 9.626 0.1105 0.512978 8.28 18.544 15.620 38.285 8.3 6.8 COTE L:CEAEU ITR FTEGAAABASIN GUALALA THE OF HISTORY CRETACEOUS AL.: ET SCHOTT JOGD-7 RS92SP14 150 1.930 148.5 0.0376 0.7034647 0.70338 0.362 0.913 0.2393 0.512961 5.49 JOGD-8 RS92SP16 145 18.179 371.6 0.1415 0.7036771 0.70339 4.922 18.072 0.1645 0.512989 7.44 18.472 15.589 38.133 8.9 7.4 JOGD-9 RS92SP26 158.5 7.661 193.4 0.1146 0.7045792 0.70432 1.366 4.004 0.2060 0.512981 6.50 18.264 15.563 37.927 8.4 6.9 JOGD-10 CJ-829-2A 150 9.347 179.3 0.1508 0.705493 0.70517 JOGD-11 CJ-829-2D 150 30.549 480.1 0.1841 0.703959 0.70357 2.953 13.195 0.1351 0.512926 6.80 JOGD-12 CJ-829-3A 150 JOGD-13 RS92SP27 150 4.746 296.6 0.0463 0.7037787 0.70368 1.020 3.564 0.1728 0.512989 7.30 18.287 15.608 38.006 8.5 7.0 JOGD-14 RS92SP30 144 21.751 341.2 0.1844 0.7039991 0.70362 2.491 11.570 0.1300 0.512888 6.10 JOGD-15 RS92SP31 150 6.748 214.6 0.0910 0.7039415 0.70375 0.941 2.784 0.2040 0.513051 7.93 JOGD-16 RS92SP32 150 18.543 341.6 0.1570 0.7039699 0.70364 1.330 4.706 0.1707 0.512953 6.65 18.189 15.387 37.299 10.5 9.0 JOGD-17 ABc-1 150 17 125 0.3935 0.70465 0.70381 6.6 47 JOGD-18 SR-3 150 20 153 0.3782 0.70458 0.70377 6.3 53
3o 22 of 13 Cretaceous Continental Granitic & Rhyolitic Clasts (KCGR) KCGR-1 RS92SP15 85 99.906 481.6 0.6005 0.7085639 0.70784 4.437 25.973 0.1031 0.512240 6.75 19.549 15.801 39.196 12.4 10.9 KCGR-2 CJ-829-3A 100 97.158 204.2 1.3774 0.709979 0.70802 5.520 24.983 0.1334 0.512388 4.07 KCGR-3 CJ-829-4B 100 88.291 210.9 1.2117 0.707975 0.70625 4.972 27.591 0.1088 0.512528 1.02 KCGR-4 RS92SP40 101 108.278 146.5 2.1389 0.7091769 0.70611 4.105 21.315 0.1163 0.512520 1.27 19.005 15.655 38.690 12.5 11.0 KCGR-5 RS92SP42 100 112.819 191.3 1.7075 0.7107051 0.70828 5.075 24.496 0.1251 0.512366 4.40 19.514 15.795 39.183 12.5 11.0 KCGR-6 RS92SP43 100 200.000 13.0 41.4442 7.857 18.128 0.2617 0.512429 4.91 19.406 15.759 38.995 12.3 10.8 KCGR-7 RS92SP45 101 120.640 102.3 3.4165 0.7134254 0.70852 4.940 25.270 0.1180 0.512388 3.87 19.428 15.776 39.074 11.1 9.6 KCGR-8 RS92SP54 100 105.044 104.6 2.9077 0.7110146 0.70688 7.018 32.909 0.1288 0.512405 3.69 19.764 15.791 39.413 11.4 9.9 KCGR-9 RS92SP6 125 85.166 177.6 1.3883 0.709005 0.70654 2.587 13.151 0.1187 0.512643 1.33 18.906 15.701 38.769 10.5 9.0 KCGR-10 RS92SP7 100 241.663 80.2 8.7297 0.7202991 0.70789 2.120 13.237 0.0967 0.512396 3.45 19.303 15.733 38.939 KCGR-11 RS92SP8 100 145.791 128.7 3.2791 0.7130564 0.70840 3.490 19.405 0.1086 0.512242 6.61 19.453 15.728 38.996 11.4 9.9 KCGR-12 RS92SP10 100 123.000 289.0 1.1465 4.578 25.515 0.1083 0.512399 3.54 19.361 15.744 39.075 9.9 8.4 KCGR-13 RS92SP55 97 143.515 21.9 19.0032 0.7334738 0.70728 2.899 18.390 0.0952 0.512388 3.62 19.418 15.742 39.037 12.6 11.1 KCGR-14 RS92SP56 100 124.534 39.0 9.2653 0.7221306 0.70896 4.721 25.614 0.1113 0.512421 3.15 19.395 15.776 39.171 12.6 11.1 KCGR-15 RS92SP58 106 160.086 128.1 3.6198 0.7133311 0.70788 2.336 15.360 0.0918 0.512432 2.59 19.606 15.793 39.361 10.8 9.3 KCGR-16 RS92SP60 100 108.417 48.5 6.4787 0.717482 0.70828 5.008 21.100 0.1433 0.512458 2.83 19.501 15.803 39.185 10.7 9.2 KCGR-17 RS92SP62 95 172.848 155.6 3.2175 0.7127644 0.70842 2.792 17.608 0.0957 0.512361 4.18 19.267 15.686 38.821 12.7 11.2 KCGR-18 RS92SP70 98 187.074 36.5 14.8856 0.7298199 0.70909 2.939 16.632 0.1067 0.512404 3.44 19.321 15.693 38.897 9.7 8.2
Eagle Rest Peak (ERP) - Gold Hill (GH) - Logan Quarry (LQ) Mafic Complex ERP-1 RS92ER1 160 1.863 168.0 0.0321 0.7030329 0.70296 3.034 9.091 0.2015 0.513090 8.72 ERP-2 CJ-40A 160 0.283 0.658 0.2597 0.512965 5.09 4.5 ERP-3 CJ-41A 160 16.000 141.7 0.3267 0.70317 0.70243 0.785 1.864 0.2544 0.513020 6.28 4.8 59 ERP-4 CJ-41A-A 160 6.149 153.0 0.1163 0.703136 0.70287 0.536 1.339 0.2418 0.513003 6.20 ERP-5 CJ-42A 160 15.000 143.7 0.3020 0.702652 0.70197 0.463 0.978 5.6 43
ERP-6 CJ-42B 160 5.8 B02204 ERP-7 CJ-43A 160 2.408 145.7 0.0478 0.702808 0.70270 0.445 1.000 0.2688 0.513069 6.94 ERP-8 CJ-43C 160 23.000 269.0 0.2474 0.703149 0.70259 5.7 43 ERP-9 RS92ER2 160 6.000 133.0 0.1215 B02204
Table 4. (continued)
87 87 87 147 143 206 207 208 18 Field Age, Rb, Sr, Rb Sr Sr Sm, Nd, Sm Nd Pb Pb Pb d O 86 Measured 86 Initial 86 144 Measured 144 204 204 204 Sample Sample Ma ppm ppm Sr Sr Sr ppm ppm Nd Nd Nd (t) Pb Pb Pb Quartz Magma WR dDWR ERP-10 RS92ER9 160 7.000 180.0 0.1048 COTE L:CEAEU ITR FTEGAAABASIN GUALALA THE OF HISTORY CRETACEOUS AL.: ET SCHOTT ERP-11 RS92ER15 160 9.000 177.0 0.1370 9.0 7.5 ERP-12 RS92ER18 160 1.000 18.0 0.1497 ERP-13 RS92ER28 160 7.000 160.0 0.1179 ERP-14 RS92SC1 160 16.000 141.0 0.3057 8.6 7.1 ERP-15 RS92SC7 160 13.721 150.9 0.2630 0.7040478 0.70345 1.137 3.833 0.1791 0.512964 6.73 8.7 7.2 ERP-16 DR-1169A 160 14.000 179.0 0.2263 0.7034 0.70289 5.3 45 ERP-17 DR-1161C 160 12.000 180.0 0.1929 0.70333 0.70289 5.7 59 GH-1 GH-1 (CJ) 160 12.000 112.0 0.3100 0.70378 0.70307 5.6 45 GH-2 SH-4 (CJ) 160 6.5 48 GH-3 CJ-21 GH 160 2.603 118.0 0.0638 0.703732 0.70359 1.394 4.340 0.1939 0.513022 7.55 5.6 53 GH-4 CJ-22 160 5.7 GH-5 CJ-24 160 6.1 GH-6 CJ-25 160 27.000 285.0 0.2741 0.7033 0.70268 6.0 53
4o 22 of 14 GH-7 CJ-27 160 6.8 GH-8 CJ-28 GH 160 2.968 156.2 0.0550 0.703618 0.70349 0.752 2.279 5.7 57 GH-9 CJ-29 160 6.4 GH-10 RS92CV5 160 3.000 106.0 0.0762 GH-11 RS92CV6 160 3.000 106.0 0.0762 8.9 7.4 GH-12 DR-139 160 18.000 215.0 7.7 47 GH-13 DR-1062 160 14.000 106.0 6.8 50 LQ-1 LQ-1 (CJ) 160 6.1 51 LQ-2 CJ-1F 160 6.2 42 LQ-3 CJ-1UF LQ 160 18.000 129.0 0.4037 0.70272 0.70180 5.8 56 LQ-4 CJ-2 LQ 160 3.282 174.5 0.0544 0.70337 0.70325 1.635 4.730 0.2087 0.513049 7.78 5.8 44 LQ-5 CJ-3 160 LQ-6 SH-6 (CJ) 160 17.000 143.0 0.3439 0.70361 0.70283 5.9 36 LQ-7 SJB-1 (CJ) 160 LQ-8 RS92CH1 160 12.000 213.0 0.1518 8.5 7.0 LQ-9 RS92SJ1 160 1.000 135.0 0.0200 LQ-10 DR-588A 160 13 153 4.1 53 a 87 86 Rb, Sr, Sm, and Nd concentrations are determined by isotope dilution mass spectrometry at University of Wisconsin-Madison. Initial Sr/ Sr and Nd (t) are calculated from measured whole rock values using the U/Pb zircon crystallization age determined for each sample; all mafic basement terrane (ERP-GH-LQ) samples are reduced to 160 Ma. Pb is corrected for mass fractionation of 0.1% per amu. Sr mass fractionation 86 88 146 144 18 18 corrected to Sr/ Sr = 0.1194 and Nd mass fractionation corrected to Nd/ Nd = 0.7219. d Omagma (per mil relative to SMOW) is calculated as d Oqtz 1.5%. B02204 B02204 SCHOTT ET AL.: CRETACEOUS HISTORY OF THE GUALALA BASIN B02204
Logan Gabbro, separated by the Vergeles-Zayante fault [Ross, 1984]. 4.1.1. Correlation of Continental (KCGR) Clasts [22] The most compelling similarities between the KCGR clasts and granites of the reconstructed Sierran tail region are their continental isotopic compositions. The chemical and isotopic compositions of the KCGR clasts (eastern Cordillera arc) makes it unlikely that sedimentary processes alone were responsible for transport of coarse material from an inboard segment of the batholithic belt to the forearc without incorporating a significant amount of material from the axial and outboard portions of the arc; such processes would produce clasts with a continuous range of affinities between continental and oceanic. The forearc depositional setting of the KCGR clasts, their eastern arc compositions, as well as the absence of detritus that has intermediate compositions, appears to require a tectonically displaced source terrane. [23] Although isotopic compositions of the KCGR clasts and the Salinia-Mojave segment of the arc are very Figure 3. Tera and Wasserburg [1972] U/Pb concordia similar, a few distinguishing age and textural character- plot of KCGR clast zircons. Numbers refer to sample istics require explanation. The presence of devitrified numbers that have ‘‘KCGR’’ prefix (see Table 2). Age rhyolites and aphanitic granite porphyries suggests that uncertainties are significantly smaller than the symbol size the KCGR clasts represent a more shallow level of the for most zircon fractions. Zircon fractions from most clasts crust than granites that are currently exposed in the have complex discordance patterns reflecting both inheri- Salinian block, western Mojave Desert, and the southern- tance (e.g., KCGR-1, KCGR-8, and KCGR-11) and Pb loss most Sierran tail [Schott and Johnson, 1998a]. This (e.g., KCGR-13 and KCGR-18). The majority of KCGR difference in crustal exposure level is readily explained clasts are interpreted to have crystallized at circa 100 Ma. by additional post-Cretaceous uplift and erosion of the Data are from Table 2. Salinia-Mojave source terranes [Pickett and Saleeby, 1993]. The age range of KCGR clasts corresponds broadly to that of the California batholiths, although the eastern batholithic character are thrust over autochthonous 125 Ma age for clast KCGR-9 is unusual for a rock that gabbroic rocks that have oceanic affinities along the has an initial 87Sr/86Sr > 0.706; most of the early Pastoria thrust [Ross, 1989]. In the Gabilan Range (now Cretaceous plutons in the batholith are intruded into the displaced 315 km to the northwest in the central western side of the arc and have initial 87Sr/86Sr < 0.706 Salinian block), an analogous relation exists between [Chen and Moore, 1982; Chen and Tilton, 1991]. In the continental granitic rocks of the range and the oceanic addition, the estimated 85 Ma age for clast KCGR-1
Figure 4. Conventional U/Pb concordia plots of JOGD clast zircon fractions: (a) 145 Ma clast ages and (b) 160 Ma clast and basement ages. Data are from Table 2.
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Mountains [Snoke et al., 1982] have the appropriate age and general composition, but there is no evidence that these complexes (where currently exposed) are of suffi- cient areal extent or proximity to the forearc to be a viable source for large clasts. [25] Correlation of Gualala gabbroic clasts with the mafic complex at Eagle Rest Peak and its offset counterparts at Logan Quarry and Gold Hill (ERP-LQ-GH) was proposed by Ross [1970] and Ross et al. [1973] but questioned by James et al. [1993]. Neither the JOGD clasts nor the rocks at ERP-LQ-GH is enriched in TiO2 with increasing FeO*/ MgO, suggesting that both originated in a primitive oceanic arc setting. Oceanic Nd isotope compositions, as well as H- O isotope variations that indicate seawater hydrothermal alteration, are also common to ERP-GH-LQ and Gualala. Given the inferred primary crystallization age ( 165 Ma) for the mafic clasts studied by James et al. [1993] and the ages for the JOGD clasts determined in this study (140– 159 Ma), it is likely that ages of the source terrane(s) of the Figure 5. SiO2 versus (Na2O+K2O) (major elements mafic Gualala clasts are quite variable. Although the ages normalized to 100% anhydrous). KCGR clasts (dia- for ERP-LQ-GH overlap those of the Gualala mafic clasts, monds); JOGD gabbroic clasts (open pluses); JOGD we concur with all previous workers [Ross, 1970; Ross et granitoid clast (pluses); and Cretaceous Cordilleran al., 1973; James et al., 1993] that ERP-LQ-GH is probably granitoids (small shaded circles; data sources given by a small surface exposure of a widespread mafic basement Schott and Johnson [2001]). Note that KCGR clasts are terrane that was exposed as the source for gabbroic detritus unusually SiO2-rich compared to most Cretaceous Cor- during the Late Cretaceous that was subsequently covered dilleran granitoids; however, there is no depletion in the by younger sediments. This mafic basement terrane is alkalis that would suggest alkali mobility. Data for this presumed to underlie portions of the southern San Joaquin study are given in Table 3. Valley, northern Salinian block, and western Mojave Desert [Ross and McCulloch, 1979; Ross, 1989], and may have been quite extensively exposed during the Late Cretaceous, (projected lower intercept) is remarkable in that it is which may decrease the apparent 100–200 km travel relatively close to the depositional age of the rock (Maastrichtian, probably about 75 Ma), suggesting rapid uplift, erosion, and displacement of the parent body. Granitic rocks of this age form the last voluminous magmatic pulse in the Salinian block and eastern Sierra Nevada Batholith [Stern et al., 1981; Chen and Moore, 1982; Mattinson, 1990; Coleman et al., 1992]. 4.1.2. Correlation of Oceanic (JOGD) Clasts [24] Mafic terranes exposed in the California Coast Ranges, western Sierra Nevada, or Klamath Mountains are unlikely sources for the JOGD clasts. Although the Coast Range Ophiolite is a good match for the JOGD clasts in terms of age [Hopson et al., 1981] and geochem- ical affinities [Shervais, 1990], none of the clasts at Gualala could represent the high levels of an ophiolite, which would include rock types such as cherts or shales, or igneous rocks such as pillow basalts and sheeted dikes. Furthermore, the Coast Range Ophiolite is overlain by Upper Jurassic through Upper Cretaceous sediments of the Great Valley group, and was probably not exposed at the time of deposition of the Gualala Formation [Ingersoll, 1983]. The Franciscan Complex is inappropriate as a Figure 6. FeO*/MgO versus TiO2. Symbols are as in source for three reasons: it lacks significant exposures of Figure 5. JOGD clasts have a compositional range very gabbro [Shervais, 1990]; its MORB-like geochemical char- similar to the mafic basement terrane at ERP-LQ-GH. Both acteristics are unlike the oceanic arc compositions of the lack TiO2 enrichment with increasing FeO*/MgO ratio that JOGD clasts [Shervais, 1990]; and finally, distinctive is characteristic of MORB. Data for this study are from Franciscan lithologies such as chert, serpentinite, and Table 3. Fields for Coast Range Ophiolite and Franciscan jadeite-bearing metamorphic rocks are absent in the Gua- are from Shervais [1990]. Data sources for Cretaceous lala clast assemblage [Wentworth, 1966]. Peridotite to Cordilleran granitoids are given by Schott and Johnson diorite complexes of the Sierra Nevada and Klamath [2001].
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Figure 7. Chondrite normalized REE diagrams for (a) KCRG clasts, (b) JOGD clasts, and (c) ERP-GH- LQ mafic complex. See discussion in text. Data are from Table 3. distances for the mafic Gualala clasts inferred from palins- topography in the source area due to continued erosion or pastic reconstructions (Figure 11). relative transport of the basin away from the Late Creta- ceous sediment source area. The presence of a first-gener- 4.2. Tectonic Controls on Late Cretaceous-Paleocene ation clast of Late Jurassic ( 145 Ma) oceanic gabbro in the Sedimentation overlying Eocene German Rancho Formation [Schott and [26] Late Cretaceous to Paleocene sedimentation in the Johnson, 2001] suggests that the same gabbroic source Gualala basin is inferred to reflect tectonic events associated terrane contributed sediment to the basin during both the with the collapse of the Salinia-Mojave segment of the Maastrichtian and early Eocene, implying that translation of Cretaceous Cordilleran arc [Schott and Johnson, 1998a]. The basal Stewarts Point Member conglomerate of Campa- nian age is a massive cobble- and boulder-dominated assemblage that is up to one kilometer thick; KCGR clasts comprise the majority of the clast population. Initiation of KCGR conglomerate sedimentation in the basin has been interpreted to reflect the tectonic emplacement of an alloch- thon that was derived from the inboard portions of the Cordilleran arc, into a position that was adjacent to the forearc [Schott and Johnson, 1998a, 1998b]. Initiation of the arc collapse event is constrained to be circa 80 Ma [Schott and Johnson, 1998a, 1998b]. [27] Data for the JOGD clasts permit two possible source terranes that allow differing tectonic interpretations. One possibility is that the upsection increase in the proportion of JOGD clasts reflects progressive erosional unroofing of the granitic allochthon and exposure of the tectonically under- lying Jurassic mafic basement terrane. This mafic basement terrane may be the sole source for Anchor Bay gabbroic clasts if its currently buried portions include circa 145 Ma rocks (similar to the Coast Range Ophiolite). Alternatively, the Anchor Bay gabbroic clasts may be derived from an Figure 8. The dD-d18O variations for whole rock ERP- oceanic arc terrane that was exotic to North America. In the LQ-GH basement samples and two JOGD clasts. All dD exotic arc model, the upsection increase in gabbroic clasts in values are significantly higher than those of primary the Gualala Formation may be the result of accretion of this igneous compositions, indicating hydrothermal interaction arc to the North American continental margin during the with fluids that contained high-dD values, such as seawater latest Cretaceous to Paleocene. Finally, it is possible that the (dD 0%), rather than meteoric waters, which have lower mafic conglomerate at Gualala contains both circa 160 Ma dD values. A seawater hydrothermal alteration trend is North American basement (ERP-GH-LQ) and circa 145 Ma shared among ERP, GH, and LQ, as well as the two JOGD exotic arc components. clasts. Data for samples from this study are from Table 4. [28] The overall decrease in conglomerate abundance and Field for the Coast Range Ophiolite is from Magaritz and clast size upward through the Late Cretaceous and early Taylor [1976]. Small gray circles are data from Salinian Tertiary sections at Gualala may reflect a decrease in plutons from Masi et al. [1981].
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Figure 10. Pb isotope variation diagrams: (a) 207Pb/204Pb versus 206Pb/204Pb and (b) 208Pb/204Pb versus 206Pb/204Pb. Symbols are as in Figure 5. JOGD clasts illustrate a relatively steep trend in that may reflect assimilation and mixing with an older Pb component that had a low Th/U ratio. S/K is Stacey and Kramers [1975] average crustal growth curve. NHRL is Northern Hemisphere reference line [Hart, 1984]. The moderately high 207Pb/204Pb ratios of the KCRG clasts overlap those of the Salinia-western Mojave plutons. Data for samples from this study are from Table 4. Data sources for Cretaceous plutonic rocks are given by Schott and Johnson [2001].
the basin during the early Paleogene was minimal (less than tens of kilometers). 4.3. Pre-Neogene Basement Relations in the Northern Salinian Block––Southern Sierran Nevada Batholith––Western Mojave Desert Region [29] The subsurface extent, age, and tectonic affinity of the mafic basement terrane that is exposed in ERP-LQ-GH are uncertain, although the range in age of up to 25 m.y. of the mafic Gualala clasts suggests the source region(s) must have been more extensive than the exposed extent of
Figure 9. Variations initial 87Sr/86Sr ratios for Gualala clasts relative to (a) U/Pb zircon ages, (b) d18O values, 206 204 (c) eNd(t) values, and (d) Pb/ Pb ratios. Symbols are as in Figure 5. Note particularly the unusual enrichment in 87 86 Sr/ Srinit at a given eNd(t), a feature that is relatively unique to KCGR clasts and basement samples of allochtho- nous granites from the Salinian block and western Mojave Desert. JOGD clasts are generally a good match for the oceanic basement terrane at ERP-LQ-GH, though some 87 86 JOGD clasts may have enriched Sr/ Srinit ratios due to limited hydrothermal alteration by seawater. Data for samples from this study are from Table 4.
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Figure 11. Pre-Neogene palinspastic reconstruction of the southernmost Sierra Nevada Batholith (SNB), north central Salinian block (SB), and westernmost Mojave Desert (WMD) region illustrating the juxtaposition of interior arc continental granitoids of the Salinia-western Mojave Allochthon (hachured pattern) with 145–160 Ma oceanic basement (stippled pattern) in a location adjacent to the forearc depositional location of the Gualala basin. The Pastoria thrust and Vergeles-Zayante fault mark this tectonic boundary in the southernmost SNB and northern Salinian block, respectively. North is generally toward the top of the diagram but cannot be well constrained. Schematic locations of faults with Neogene offset are abbreviated as follows: SAF, San Andreas fault; GF, Garlock Fault; SGHF, San Gregorio- Hosgri fault; RRF, Reliz-Rinconada fault; BF, Butano fault. Modified from James [1992] and Powell [1993]. Granitic basement in the Salinian block: BH, Bodega Head; MM, Montara Mountain; BL, Ben Lomond; PR, Point Reyes; GR, Gabilan Range. Mafic basement terrane exposures: LQ, Logan Quarry; GH, Gold Hill; ERP, Eagle Rest Peak; AP, Antimony Peak.
ERP-LQ-GH. Extension of the ERP-LQ-GH terrane may be related structures were subsequently modified by Neogene reflected in the strong positive magnetic anomaly that deformation associated with the San Andreas fault system extends westward beneath the Butano basin in the Salinian [Ross, 1984, 1989]. block, and north and east beneath the southern San Joaquin [31] The tectonic reconstruction favored by our data Valley [Jachens et al., 1998]. Mafic basement is inferred to (Figure 11) is at odds with an inferred low-latitude origin underlie the granites of the central Salinian block at a depth for the Gualala basin [Kanter and Debiche, 1985], where of 13 km [Ross and McCulloch, 1979]. No samples that shallow magnetic inclinations suggested between 4000 and have ages of 145 Ma have yet been recovered from the 1800 km of northward transport of the basin relative to buried portions of this terrane, but the mafic tonalite of cratonic North America. These magnitudes of transport are Antimony Peak [Ross, 1989], located in the westernmost far in excess of those suggested by the provenance data Sierran tail and structurally below the Pastoria thrust just a presented here ( 450–650 km), even after a component of few kilometers south of the Eagle Rest Peak (Figure 11), has northward motion east of the San Andreas fault system been dated at 131 Ma [James, 1986]. [Dickinson and Wernicke, 1997] is taken into account. We [30] Conglomerate provenance data presented here con- therefore suggest that the paleomagnetic data from the strain the depositional location of the basin to have been Gualala basin overestimate the extent of northward transport adjacent to North America at the latitude of the southernmost of the basin. Uncertainties due to compaction and deforma- Sierra Nevada (and palinspastically restored northern Sali- tion seem likely explanations for the excessive transport nian block) during the Late Cretaceous. The depositional distances inferred from paleomagnetic data [e.g., James et location of the Gualala basin is only broadly constrained, al., 1993; Kodama and Davi, 1995]. Paleontologic evidence however, because the age and compositional variability of that implies a low-latitude Upper Cretaceous origin for beds the mafic basement terrane, as well as the Late Cretaceous in the Gualala Formation includes the rudistid bivalve location of its tectonic boundary with Salinian granites, are Coralliochama orcutt, which has subtropical Tethyan affin- poorly known. The contact between the allochthonous ities [Elder et al., 1998]. Such evidence is unlikely to place continental granite-rhyolite terrane and the underlying oce- rigorous constraints on paleolatitudes because the rudist anic mafic terrane may have been a laterally extensive, fragments in the Anchor Bay member conglomerate beds originally subhorizontal detachment surface [Silver, 1982, are detrital in origin, and the same beds also contain 1983; Silver and Mattinson, 1986]. Remnants of this terrane Paleocene foraminifera [McDougall, 1998]. boundary are preserved in the Sierran tail as the Pastoria thrust and in the Salinian block as the Vergeles-Zayante fault [Ross, 1984, 1989]. Our results therefore suggest that an 5. Summary Upper Cretaceous detachment in the Sierran batholith best [32] The Late Cretaceous depositional location of the explains the two populations of Gualala clasts, and this and Gualala basin appears to reflect the most outboard or
19 of 22 B02204 SCHOTT ET AL.: CRETACEOUS HISTORY OF THE GUALALA BASIN B02204 westernmost record of sedimentation in the Cordilleran arc Church, S. E. (1976), The Cascade Mountains revisited: A re-evaluation complex of California after cessation of Sierra Nevada in light of new lead isotopic data, Earth Planet. Sci. Lett., 29, 175– 188. batholith magmatism and, as such, potentially records tec- Coleman, D. C., T. P. Frost, and A. F. Glazner (1992), Evidence from the tonic events whose record is not preserved in the major, Lamarck Granodiorite for rapid Late Cretaceous crust formation in Cali- interior portions of the arc. Detailed U/Pb zircon ages, fornia, Science, 258, 1924–1926. Cowan, D. S. (1994), Alternative hypotheses for the Mid-Cretaceous geochemical, and isotopic data for conglomerate clasts iden- paleogeography of the Western Cordillera, GSA Today, 4, 181, 184– tify two groups that were deposited contemporaneously: 186. (1) the KCGR suite comprising granitic and rhyolitic clasts Cowan, D. S., M. T. Brandon, and J. I. Garver (1997), Geological tests of hypotheses for large coastwise displacements: A critique illustrated that have mid-Cretaceous ages and continental chemical and by the Baja British Columbia controversy, Am. J. Sci., 297,117– isotopic compositions, that were derived from a fragment of 173. the shallow levels of the eastern (interior) portion of the DePaolo, D. J. (1981), A neodymium and strontium isotopic study of the Cretaceous Cordilleran magmatic arc, and (2) the JOGD suite Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California, J. Geophys. Res., 86(B11), 10,470 – comprising gabbroic and quartz dioritic clasts that have Late 10,488. Jurassic crystallization ages and chemical and isotopic com- Dickinson, W. R., and R. F. Butler (1998), Coastal and Baja Califor- positions that indicate derivation from an oceanic arc. The nia paleomagnetism reconsidered, Geol. Soc. Am. Bull., 110, 1268– 1280. strong isotopic and compositional contrast between these Dickinson, W. R., and B. P. Wernicke (1997), Reconciliation of San suites and their common occurrence in individual beds imply Andreas slip discrepancy by a combination of interior Basin and Range a tectonic juxtaposition of their sources in a near-forearc extension and transrotation near the coast, Geology, 25, 663–665. Dodge, F. C. W., B. P. Fabbi, and D. C. Ross (1970), Potassium and setting prior to sedimentation, which most likely occurred rubidium in granitic rocks of central California, U.S. Geol. Surv. Prof. through westward collapse of the eastern Sierra-Mojave arc Pap., 700D, 108–115. along a major detachment structure. The structures that were Elder, W. P., L. R. Saul, and C. L. Powell II (1998), Late Cretaceous and associated with juxtaposition of the eastern continental and Paleogene molluscan fossils of the Gualala block and their paleogeo- graphic implications, in Geology and Tectonics of the Gualala Block, western oceanic sections of the arc are likely to have Northern California,editedbyW.P.Elder,Publ. 84, pp. 149–167, produced weaknesses in the lithosphere that were later Pac. Sect., SEPM, Golden Gate, Calif. exploited by transform motion along the San Andreas sys- Fleck, R. J., and R. E. Criss (1985), Strontium and oxygen isotopic varia- tions in Mesozoic and Tertiary plutons of central Idaho, Contrib. Mineral. tem. Correlation of the Late Cretaceous conglomerate clast Petrol., 90, 291–308. assemblage with a source on the North American continent at Graham, S. A., R. G. Stanley, J. V. Bent, and J. B. Carter (1989), Oligocene the latitude of the southernmost Sierra Nevada provides and Miocene paleogeography of central California and displacement along the San Andreas Fault, Geol. Soc. Am. Bull., 101, 711–730. strong evidence that the terranes of western California that Hart, S. R. (1984), A large-scale isotope anomaly in the Southern Hemi- lie west of the San Andreas transform system are not exotic or sphere mantle, Nature, 309, 753–757. far traveled, contrary to paleomagnetic data that imply Hart, S. R., D. C. Gerlach, and W. M. White (1986), A possible new Sr-Nd- thousands of kilometers of northward transport since the Pb mantle array and consequences for mantle mixing, Geochim. Cosmo- chim. Acta, 50, 1551–1557. Eocene. Hawkins, J. W., S. H. Bloomer, C. A. Evans, and J. T. Melchior (1984), Evolution of intra-oceanic arc-trench systems, Tectonophysics, 102, 175– 205. Hopson, C. A., J. M. Mattinson, and E. A. Pessagno Jr. (1981), Coast [33] Acknowledgments. Supported by Geological Society of Amer- ica research grant 5045-92, Sigma Xi grant-in-aid of research 9203, and Range ophiolite, western California, in The Geotectonic Development UW-Madison/Chevron Foundation graduate research fund grant 133-J392 of California, Rubey Vol. I, edited by W. G. Ernst, pp. 418–510, Pre- to R.C.S. and National Science Foundation grant EAR-96-28549 to ntice-Hall, Old Tappan, N. J. C.M.J. Initial field and laboratory work by C.M.J. and J.R.O. was funded Ingersoll, R. V. (1983), Petrofacies and provenance of late Mesozoic forearc by the U.S. Geological Survey. We thank Don Ross, Carl Wentworth, Bob basin, northern and central California, AAPG Bull., 67, 1125–1142. McLaughlin, Allison Reitz, and Eric James for valuable discussions. Jachens, R. C., C. M. Wentworth, and R. J. McLaughlin (1998), Pre-San S. Boardman and E. H. Christiansen provided major and trace element Andreas location of the Gualala block inferred from magnetic and gravity XRF analyses. 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